A NOVEL OPTICAL STUDY OF THE TUNNELING RATE AND AMBIPOLAR TRANSPORT CHARACTERISTICS OF EXCITONS IN SUPERLATTICES WITH A VARIETY OF BARRIER THICKNESSES

We report the use of a time-resolved transient-grating technique to st udy the in-plane ambipolar diffusion coefficient and interwell tunnell ing probability for excitons in a number of multiple quantum well samp les. Our specimens were selected with barrier thicknesses ranging from 14 to 141 angstrom thus giving complete coverage of the transition fr om isolated quantum well to superlattice behaviour. Two types of MBE g rowth conditions were used giving rise to samples with low or high sur face defect density. The surface defect density had a strong effect on the in-plane mobility but not the tunnelling probability. The samples show surprisingly strong interwell coupling even with the thickest ba rrier, which we attribute to defect assisted tunnelling.